Broadband access technologies have allowed service providers to expand their content and service offerings to both business and home users. For example, a user may subscribe to multiple services or applications, such as voice service, Internet access service, video streaming, a gaming service, etc. from one or more service providers. These services and/or applications available through a private or public data network (e.g., the Internet) may be delivered over a single network connection, such as a Digital Subscriber Line (DSL) line.
Wireless local area networks (WLANs), like their wired counterparts, are being developed to provide high bandwidth to users in a limited geographical area. They are being developed as an alternative to the high installation and maintenance costs incurred by addition, deletion or modification experienced in wired infrastructures, thereby targeting the typical markets for WLANs to consumer and small business segments.
Currently, the most widely implemented WLAN standard is the IEEE 802.11 family of standards, which has been released with different specifications. In particular, the 802.11b specification, also known as Wi-Fi, operates in the 2.4-GHz frequency range and uses direct sequence spread spectrum technology, whereas the 802.11g specification operates in the 2.4-GHz frequency range and uses orthogonal frequency division multiplexing (OFDM) technology. On the other hand, the 802.11a specification operates in the 5.15-5.35 GHz and 5.725-5.925 GHz.
WLANs operate by employing wireless access points (or access gateways) that provide users having wireless “client” devices in proximity to the access point, with access to various types of data networks such as an Ethernet network or the Internet. The wireless access points include at least a radio that typically operates according to one of the standards specified in different sections of the IEEE 802.11 specification. Generally, radios in the access points communicate with client devices by utilizing omni-directional antennas that allow the radios to communicate, i.e., to transmit and receive data, with client devices in any direction. The access points are then connected (by hardwired connections) to a data network system that completes the access of the client device to the data network.
Wireless gateways are then usually implemented as the final link between the existing wired network and a group of clients, giving these users wireless access to the full resources and services of the data network across a building.
US patent application No. 2002/0175864 discloses an electronic device for bridging a mobile device to a wired network comprising an antenna pair consisting of two mutually orthogonal slot antennas formed on a metallic strip and sharing a common portion of the strip as the grounding unit. The antenna pair can be concealed inside the electronic device.
The main causes of performance degradation in wireless communications are multipath fading, polarization mismatch and the co-channel interference. One technique used to increase signal efficiency is Multiple Input Multiple Output (“MIMO”). MIMO techniques use a plurality of antennas coupled to a signal processing chipset for the simultaneous transmission and/or reception of multiple signals. MIMO provides antenna diversity against undesirable path effects and improves communication channel capacity. As transmission paths between the transmit antennas and receive antennas are generally linearly independent, the probability of successful transmission of signal to a client generally increases in proportion to the number of antennas.
In wireless communication employing MIMO, two or more unique data streams are transmitted and received through one radio channel whereby two or more times the data rate per channel are delivered. More than one coherent radio up-converter and antenna are used to transmit the multiple signals, and more than one coherent radio down-converter and antenna receive the multiple signals. Peak throughput in MIMO systems increases by a factor equal to the number of data streams transmitted (or received) in the channel. Because multiple signals are being transmitted (or received) from a different radio and antenna, MIMO signals are often called “multi-dimensional” signals.
WO patent application No. 2006/003416 describes a wireless network device having orthogonally polarized antennas arranged to provide transmit and/or receive polarization diversity. The antennas may be integrated within or mounted on a housing of the device so that they do not physically project outside the housing. The device may have two such antennas, arranged orthogonally on the same or different faces of the housing.
MIMO techniques can work in transmission diversity and/or reception diversity. According to the common terminology, a MIMO technique works in both transmission and reception diversity. A system which uses multiple antennas at the transmitter and a single antenna at the receiver is named as Multiple Input Single Output (MISO), whereas a system using a single antenna at the transmitter and multiple antennas at the receiver is named as Single Input Multiple Output (SIMO). In reception diversity (i.e., MIMO and SIMO techniques), the responses of the multiple antennas at the receiver side are combined.
As previously mentioned, the diversity works well when fluctuations due to multipath fading of the received signal at the multiple receiver antennas are independent of each other. Antenna diversity techniques can take various forms, for example, space, polarization, frequency, or angle diversity. In space diversity scheme, the diversity antennas have to be placed apart in various spacings.
A review of polarization diversity schemes in wireless communication is reported in “Investigations on Polarization Schemes for use in Wireless Environments”, by M. Kar and P. Wahid, published in the Proceedings of SPIE, vol. 4740 (2002), pages 160-167.
S. B. Yeap at al. in “Integrated diversity antenna for laptop and PDA in a MIMO system”, published in IEE Proc.-Microw. Antennas Propag., vol. 152, no. 6 (2005), introduces a design of double-folded dipole antenna filled with a slab of dielectric material. According to the authors, the antennas can be spatially placed close to each other (less than λ/2), while implementing polarization diversity. A laptop design is described in which two different orientations of the dielectric loaded double-folded dipole are implemented on the screen of the laptop.
A polarization diversity antenna is described in the article “A 2.4 GHz Polarization-diversity Planar Printed Dipole Antenna for WLAN and Wireless Communication Applications”, by H.-R. Chuang et al., published in Microwave Journal, vol. 45, pages 50-63 (2002), in which two orthogonal printed dipole antennas, for vertical and horizontal polarization, respectively, are combined and fabricated on a PCB substrate.